1. Field of the Invention
The present invention relates to an improvement of a digital broadcast receiving device which is used for a digital broadcasting system in which on the transmission side a broadcasting station broadcasts both video and audio signals or either one after coding or digitizing them, and which on the reception side digitally demodulates a digital broadcasting signal, corrects errors and converts the resulting signal into an analog signal for reproduction.
2. Description of the Related Art
Generally, satellite broadcasting, it is necessary to use an antenna with sharp directivity and to adjust its bearing precisely to the direction of a broadcasting satellite.
In an analog conventional satellite broadcast receiving device, the antenna bearing is adjusted so that the maximum antenna input field strength is obtained while observing the pointer of a level meter indicating a signal strength.
Generally, the above-mentioned antenna input field strength is obtained from the AGC voltage of an AGC circuit in the input stage of the receiver by conversion.
However, this method in which the input field strength only is converted from the AGC voltage and indicated on the meter has the following two disadvantages. (1) Sufficient precision and stability cannot be obtained. (2) The high input field strength does not necessarily mean good signal quality. Therefore when image quality deteriorates, namely when reproduced video and audio quality is lowered due to the deterioration of C/N (Carrier to Noise) ratio, it is impossible to know that such image quality deterioration is caused by the lowered C/N ratio of a transmission system or by a receiver.
As described above, in the reception of the satellite broadcasting signal, high input field strength does not mean an adequate receiving signal quality. The C/N ratio of the received signal is an important factor. Therefore, in some cases, an improvement is made so as to indicate not only the input field strength but also the C/N ratio to show the receiving signal quality.
For this purpose, in an analog satellite broadcast receiving device, a triangular noise proper to frequency modulation (FM) system is measured to obtain the C/N ratio of the received signal.
It is expected that broadcasting digitization will bring about higher video quality, higher audio quality, multiple broadcasting channels and multiple broadcasting programs and at the same time multimedia service and interactive service.
In a digital satellite broadcast receiving device, a modulation system different from analog system such as QPSK modulation is employed. Therefore, the C/N ratio cannot be measured and indicated by the same procedure as that of the analog system.
In Japan, the analog and digital broadcasting satellites exist in mixed form in almost the same direction. Therefore, it is impossible to determine which broadcasting system is now being received, analog system or digital system by measuring and indicating only the input field strength in the antenna bearing adjustment.